IN A NUTSHELL:Because it’s reflective and it’s lit by the glare from the daylit lunar surface. In fact it’s equally readable in the pictures taken during LM testing in Earth orbit.

THE DETAILS: There are claims that the “United States” lettering on the descent stage of the lunar module has been brightened and made more visible by retouching it in the photographs or by training a studio spotlight on it when the pictures were taken. This was done, presumably, for propaganda purposes, in order to point out that the Moon landing was an American accomplishment. With the Sun behind the LM, it is said, the logo in shadow should not be so clearly visible.

The “United States” marking (circled) is allegedly too bright in this picture, which incidentally is not an original Apollo photograph: it’s a composite made by Ed Hengeveld (Apollo Lunar Surface Journal).

The logo is indeed lit despite being on the shadow side of the spacecraft. But the lighting doesn’t come from an artificial spotlight. It comes from the sunlight reflected off the surrounding lunar surface, which is in daylight. The logo was brighter, flatter and more reflective than the surrounding skin of the vehicle and so reflected more light towards the photographer.

For example, the “United States” lettering stands out also in the photographs of the Apollo 9 mission, during which the lunar module was tested in Earth orbit, because it reflects the glare from the surface of the Earth.

Moreover, the picture that is often presented as evidence of this alleged fakery is not a real Apollo photograph. It’s a composite image, created by Ed Hengeveld and published in the Apollo Lunar Surface Journal, where it is clearly labeled as a montage. It was made by joining various photographs from the Apollo 11 mission and by adding fake portions of sky and an equally fake Sun. This is revealed not only by the fact that its author openly presents it as a whimsical fabrication, but by the mismatched perspective and by the reflections of the Sun, which are not oriented consistently. The digital compositing also distorts the perception of contrast, which is pushed far more in this montage than it is in the original pictures, enhancing the visual appearance of a strangely bright logo. The logo is not as bright in the original photographs used to create this montage.